Accessing a network resource

This is from the newer Windows 7 computer. I can access some drives on the older XP computer, but not the external hard drive known as "Q", although I could in the past. I can access other drives on the older computer.

Sometimes I can see the network form Windows Explorer from this computer and sometimes I can't. When I can't if I open another software package, such as Word, and then try to open a file on the XP computer, then I see the network, but still can't access "Q' although I could in the past.